Small-sized capacitors having large capacitance have been desired for the use in electronics such as cellular phones and personal computers. Of the capacitors, tantalum capacitors and niobium capacitors, which have large capacitance for their sizes and have good performances, have been preferably employed. Furthermore, in recent electronic devices, low-voltage, high-frequency, and/or low-noise operation is desired. In solid electrolytic capacitor, higher capacitance, low ESR (equivalent series resistance), and improved tan δ characteristics are being demanded.
For the anode of a capacitor which uses a valve-acting metal, a valve-acting metal alloy, a valve-acting metal compound or the like, in general, a porous anode made of a valve-acting metal aggregate powder, a valve-acting metal alloy aggregate powder or a valve-acting metal compound aggregate powder has been used.
For attaining high capacitance, low ESR and improved tan δ characteristics, both large specific surface area of a porous anode and good property for impregnation with a cathode agent provided as an opposite electrode. For making the specific surface area of the porous anode larger, the size of a primary powder that constitutes the anode is made small. In this case, however, a minute primary powder may inevitably create small pores. In production of any capacitor using a porous anode, a sufficient amount of a solid electrolyte needs to be efficiently impregnated into the deep parts of the respective pores from the surface of the anode. In particular, in the case of a large porous anode having a volume of 10 mm3 or more in which the distance from the surface to the deep part is long, solid electrolyte can not be sufficiently impregnated into small pores. Moreover, there is another problem that uniform pores cannot be formed owing to variation in pore size. Therefore, there has been desired a porous anode having a pore diameter distribution suitable for each kind of cathode agent to be used.
When niobium, niobium oxide, tantalum, tantalum oxide, or the like is used as a valve-acting metal, a valve-acting metal alloy or a valve-acting metal compound to be used in the anode of a capacitor, generally, a niobium aggregate powder, a niobium oxide aggregate powder, a tantalum aggregate powder, a tantalum oxide aggregate powder or the like, each having a controlled oxygen content, which is produced through reduction treatment on niobium or tantalum containing oxygen, is employed. As a method for producing these aggregate powders, reduction treatment is conventionally known.
U.S. Pat. Nos. 4,722,756, 4,960,471, JP 03-229801 A (U.S. Pat. No. 4,964,906), JP2002-507247 A (WO1998/019811), JP2002-524378 A (WO2000/015555) and JP 2002-524379 A (WO2000/015556) describe about manufacturing methods using oxygen getter metal. U.S. Pat. No. 4,537,641 and JP 2002-544375 A (WO2000/067936) describe about methods in which a reductant is gasified and then subjected to a reaction. U.S. Pat. Nos. 1,728,941 and 4,687,632 describe about manufacturing methods where a halogenated salt or the like is used as an auxiliary agent. U.S. Pat. Nos. 3,697,255, 5,242,481, GB 870930 B and JP 2002-544677 A (WO2000/069588) describe about manufacturing methods where alkali metals, alkali earth metals, rare earth metals, aluminum, carbon, or the like are used. JP 03-170648 A (U.S. Pat. No. 5,011,742) and JP 2003-13115 A describe about manufacturing methods in which a reductant and a valve-acting metal are placed apart from each other and then allowed to react. GB 1266065 B and JP 2000-119710 A (U.S. Pat. No. 6,136,062) describe about manufacturing methods in which reduction reaction is carried out in two steps at controlled temperatures. U.S. Pat. No. 2,516,863 describes about a manufacturing process in which silicon alloy or metal hydride is used as a reductant. JP 11-111575 A describes about a process of manufacturing a sintered anode in which a molded product for an anode having an implanted anode lead coexists with a reductant and then heated.
However, each of those manufacturing methods intends to control the oxygen amount without proactively controlling pore formation and therefore, when a solid electrolytic capacitor is prepared using a sintered anode obtained by any one of the above manufacturing methods, it is difficult to form pores suitable for impregnation of a cathode agent. In particular, the use of a larger sintered body (anode) of 10 mm3 or more in size results in poor impregnation of the cathode agent. As a result, the resulting capacitor has problems of its low capacitance, high ESR and increased tan δ.
JP 2001-345238 A describes a manufacturing process in which a pore-forming agent is used. In the document, magnesium, magnesium hydride, calcium, calcium hydride, aluminum and so on are exemplified as acid-soluble pore-forming agents. However, those pore-forming agents are materials which are used as reductants in the prior art documents as described above. Therefore, even if a capacitor is produced by using a powder material prepared by the method using such an acid-soluble pore-forming agent described in the document, none of the problems of low capacitance, high ESR, and increased δ can be solved just as in the case with the other prior art documents as described above.